CHAPTER VII. 



THE MARSHALLTOWN CLAY-MARL. 



This formation is more variable in its lithologic characteristics 

 than any of the others lying above the Magothy, which have been 

 mentioned. The variations in the formation are gradual, how- 

 ever, and there is no difficulty in tracing the formation from one 

 place to another. It ranges from a sandy clay with little or no 

 glauconite, to a clayey greensand marl which, in Salem County, 

 has been dug for fertilizing purposes, and which was mistaken 

 by Cook, in the early work of the Survey, for the Lower Marl, 

 now called the Navesink. In Monmouth County the formation 

 is chiefly a laminated, micaceous clay with thin seams of sand, 

 glauconite grains being absent from it, except in the upper por- 

 tion of the bed, and then the greensand is only locally conspicu- 

 ous. Near Marshalltown, in Salem county, the formation is a 

 nearly pure greensand marl, and was once extensively used for 

 fertilizer. The thickness of the formation is probably between 

 30 and 40 feet, its transition into both the subjacent and super- 

 jacent formations is somewhat abrupt, and it can be easily recog- 

 nized and mapped from Monmouth to Salem Counties wherever 

 it is not too deeply buried by the Pleistocene formations. 



FAUNA o? THE MARSHALLTOWN CLAY-MARL. 



Fossils have been collected from the Marshalltown formation 

 only in its more marly portions in the southern part of its 

 area. The only collections made by the writer have been in the 

 vicinity of Swedesboro, near which place one locality has afforded 

 a large fauna in which the specimens have their actual shells 

 perfectly preserved. This same fossiliferous horizon doubtless 

 occurs elsewhere in the region. Credner 1 undoubtedly had the 



'Zeitsch. der Deutch, Geolog. Gesell, vol. 22, p. 191 (1870). 

 6 PAL (8l) 



